The invention relates to a method and apparatus for measuring the modulus of plastic polymers, and especially of unvulcanized elastomer compounds.
The modulus of an unvulcanized elastomer compound, sometimes referred to as its "green strength", is an indication of its capacity to resist deformation in an uncured state. This property is important, for example, in the manufacture of rubber articles, when the articles are stressed before they are vulcanized. One such instance is in the manufacturing, when the components of the tire are assembled in the uncured state on a building drum, then shaped into a toroid form before final vulcanization in a mold under pressure. If the uncured compound exhibits excessive distortion under stress, certain portions of the tire may be cured in incorrect alignment with each other, and the resultant tire will be defective.
Thus, where green strength is important it is very helpful to have an accurate test for measurement of green strength, or modulus. Known techniques for measuring green strength are limited by the problems associated with applying a controlled tensile force to the sample. This is accomplished in known devices by some method of gripping the sample and then measuring its elongation under a given tensile stress. Since the polymer sample is by nature plastic, the force used in gripping it will result in distortion of the sample, with attendant inaccuracies in measurement. Similarly, known methods of measuring elongation of the sample, relying on visual observation for the most part, introduce significant errors.
Thus a need exists for a method and apparatus for quick, accurate determination of the modulus of a plastic polymer, especially an uncured vulcanizable rubber compound.